Cannot delete contacts in Exchange Admin center for on-prem users

[AtanasM]

I am not able to create new Online mailboxes, because the on-prem mail users exist as contacts in the Exchange Admin Center.

When I try to delete the contacts there, the following error messages appear:

New Exchange Admin Center: "We have a problem with our server. Please try to resubmit your request later".

Old Exchange Admin Center: "Error checking in agent 'Windows LiveId Agent': 'Unable to save. 'Admin STEP' is outside of a valid server writing range."

When I tried to delete the on-prem mailuser with the command Remove-MailUser in PowerShell, appears again the same error message:

"Error checking in agent 'Windows LiveId Agent': 'Unable to save. 'Admin STEP' is outside of a valid server writing range."

Hybrid configuration isn't set up.

[#2 · Accepted solution]

If you are certain you want to delete these objects (as in they are not using any other services within M365), use the Remove-MsolUser cmdlet. Or exclude them from the scope of dirsync.

Alternatively, if you want to "reset" their ExO status (for migration), you can try the method outlined here: https://techcommunity.microsoft.com/t5/exchange-team-blog/permanently-clear-previous-mailbox-info/ba...